Guided Radar: Contact-Based Level Measurement
update on 2011-10-31
Traditional technologies are being replaced by more modern technologies that function with the same accuracy regardless of the medium and that react to changing measurement parameters in a more robust way with a greater immunity to sources of interference. By Christoph Mueller, business unit manager, Sick AG
The importance of sensors in liquid processes found in industrial production is increasing for a variety of reasons. Firstly, modern sectors, such as the electronics and solar industries, tend to exploit considerably more wet processes than the traditional metal-processing industries. This is because these new sectors are more chemically aligned, whereas the fluid sensors of the older industries are mainly involved in hydraulic, cooling, lubrication and cleaning processes.
Secondly, the trend towards reducing the number of maintenance and commissioning personnel and towards flexible production (whereby equipping times and parameter changes take place with as little productivity loss as possible) affects fluid processes in particular and as such, the sensors involved. Last but not least, more sparing use of industrial fluids offers major potentials for reducing material consumption and cutting the amounts of hazardous wastes.
Against this background, the technological basis employed by the fluid sensors must change considerably during the coming years. Traditional measurement principles electromechanical and simple electromagnetic sensors, for example will be replaced by more modern technologies that function with the same accuracy regardless of the medium and that react to changing measurement parameters considerably more robustly and with greater immunity to sources of interference.
Time-of-flight based technologies have a key role in level measurement because their measurement results are hardly influenced by the liquid to be measured. These include both non-contact processes, such as radar and ultrasonic measurement, as well as technologies involving contact with the medium, such as guided radar.
As a result of its price positioning, but particularly also due to its robustness against environmental influences (such as foams and dusts), the technology of guided radar time domain reflectometry is of particular importance here. Whereby the fact that this technology must not be calibrated for the medium, but can merely be parameterised in a dry state, is particularly advantageous.
No subsequent recalibration is necessary when the medium is changed or there is an alteration in the tank geometry. The high tolerance towards foams and other surface effects is a further plus point compared to other technologies, such as ultrasonic or capacitive processes.
The principle of guided radar (often also described as guided microwaves) is a time-of-flight process in which a microwave pulse is coupled into a metallic probe and runs along it. If the pulse meets a change in the dielectric constant, as is found at the surface of the medium in the tank, some of the energy is reflected and received again by the sensor with a time delay. The distance of the reflection point, and as such, the level of liquid or bulk material present in the tank, is determined from the time-of-flight of this signal.
Because the time-of-flight is evaluated, the process is unaffected by media properties and as such requires no calibration. The only medium property that influences the quality, but not the applicability, of the measurement principle is the dielectric constant ?r (relative permittivity) of the medium, which affects the strength of the signal reflected by the surface interface. The measurement process can be used in large or small tanks, with differing challenges for the evaluation electronics employed.
Advantages & Limitations
While the advantage of the non-contact processes is that they do not have any direct contact with the medium and therefore, at least in theory, have a lower susceptibility regarding chemical resistance problems they are, however, limited regarding their mounting capabilities and are sensitive towards fixtures and environmental influences in the gas phases above the liquid.
Moreover, with contact-based time-of-flight measurement, a variety of probe designs expand the scope of application considerably and can substantially improve the quality of the measurement signal. As such, a coaxial design with an internal conductor that guides the signal and an external conductor that protects the signal from interference provides highly accurate measurement results, despite difficult or moving tank fixtures.
Alternatively, a simple mono-cable probe, for example a stainless steel rod with a diameter of 7 mm, offers the advantage that it is insensitive to the formation of deposits or caking in dirty media and is also easily cleaned in hygienic processes. This cleanability is also a major advantage compared to electromechanical float switches whose floats always present a cleaning problem and necessarily offer space for process-endangering nests of dirt and bacterial contamination.
While guided radar has already been an established technology for many years in the larger tanks of process automation, its use in the smaller buffer, storage and waste tanks of factory automation has only been possible up to now to a limited extent, due to insufficient accuracy and the large inactive areas. Designs and evaluation units optimised for small tanks have now opened up this new field.